In a case where a submarine tunnel is formed, it is impossible to form many shafts from which underground excavators are started. Moreover, it is difficult to discharge excavated sediment. What is even worse, there arise many risks in a case where long distance excavation is performed by a single underground excavator. Accordingly, in a case where a submarine tunnel is formed, two underground excavators are started toward each other in order to shorten the distance of excavation made by the underground excavator. Then, the tunnels respectively excavated by the two underground excavators are joined to each other under the ground.
However, if the centers of the two underground excavators are deviated laterally or vertically at the junction, the two tunnels may not join together. Furthermore, if the centers of excavation of the two excavators are not aligned, the outbreak increases undesirably, the number of the excavation operations is increased and the quantity of agent to be injected is enlarged, causing the construction cost to be raised excessively. Therefore, the centers of the excavation made by the two excavators must be aligned to each other. Accordingly, the relative position between the two underground excavators must be detected to correct the positional deviation. Hitherto, the positional deviation taken place between the two underground excavators has been corrected by detecting the positional deviation between the two underground excavators and the designed tunnel line or the position with respect to a reference point such as the start point. Thus, the error correction is performed in accordance with the positional deviation thus-obtained.
Hitherto, the position of the underground excavator positioned underground has been obtained by the following methods:
(1) The position of the underground excavator from a reference point and the deviation from the designed line are obtained by a measurement performed underground by means of a transit or the like.
(2) An optical beam transmitting device for generating coherent light such as a laser beam is disposed in the shaft from which the underground excavator is started. The designed tunnel line is irradiated with the above-described device so as to read light spots on a target attached to the underground excavator. As a result, the position, the deviation and deflection angle of the underground excavator made from the start shaft are obtained.
(3) An azimuth gyro compass, a pressure type settlement gauge, an inclination gauge and a distance meter acting by making a segment length built up in the tunnel to be a reference are combined to one another so as to obtain the relative position from the reference position.
However, each of the above-described conventional methods for obtaining the position of the underground excavator encounter the following problems, causing a difficulty to arise when accurate underground joining is required.
A problem of a practical use arises in method (1) because the real-time measurement cannot be performed since many measuring points must be required when a bent tunnel is excavated. Furthermore, in method (2) there arises a necessity of moving the optical transmitting device to a proper position if the tunnel designed line is bent because the laser beams emitted from the start shaft cannot be applied to the target. Furthermore, the laser beam cannot be directly applied to cover the overall length of the designed line. Therefore, whenever the optical transmitting device is moved, the relative positional relationship among the target, the optical measuring device and the tunnel designed line must be actually measured. Then, the designed route must be obtained by performing calculations in accordance with the result of the measurement before the position, the deviation and the deflection angle of the underground excavator are obtained. As a result, there arise a problem in that the movement of the optical transmitting device, the measuring and the calculating operation takes excessive labor, causing the efficiency in the excavating operation to be deteriorated.
In addition, method (3) is not satisfactorily used to excavate a long distance tunnel due to the generated accumulated error. Also it is not suitable in a case where a curve having a small curvature radius is excavated or a tunnel in which curves are continued is excavated. In addition, the error is further enlarged in a case, for example, the underground joining operation, in which the relative position between two underground excavators is measured.
Accordingly, the assignee of the present invention has suggested a position detection apparatus (Japanese Patent Application No. 1-223035) in which a magnetic field producer is attached to either of the two underground excavators to be joined together underground, a magnetic field detector for detecting the magnetic field produced by the magnetic field producer is attached to the remaining underground excavator, the magnetic field detector is moved closer to the magnetic field producer by a boring device. A detection signal transmitted from the magnetic field detector and the amount of excavation made by the boring device are supplied to a calculating device so that the relative position between them are obtained.
That is, the position detection apparatus disclosed in Japanese Patent Application No. 1-223035, as shown in FIG. 28, arranged in such a manner that, when two rotational excavation type shield machines (underground excavators) 10a and 10b, which have moved to confront each other while excavating sediment, approach each other at a distance of about 30 to 40 m, a boring 12 is forwardly moved from a predetermined reference pint of a small consolidation type excavator (omitted from illustration) disposed on the front surface of the shield machine 10b toward the other shield machine 10a along the center of excavation of the shield machine 10b. As a result, it is brought into contact with the front surface 14 of the other shield machine 10a.
The boring 12 has, in the leading portion (in a portion of a predetermined length which is positioned slightly away from the leading portion) thereof, a magnetic field detector 16 serving as a position detection sensor (see FIG. 29). The above-described magnetic field detector 16 comprises a horizontal directional eccentricity amount detection sensor 16a and a vertical directional eccentricity amount detection sensor 16b. The horizontal direction eccentricity amount detection sensor 16a is composed of two magnetic sensors such as coils disposed on a horizontal plane to be perpendicular to each other. The vertical directional eccentricity amount detection sensor 16b is composed of two magnetic sensors such as coils disposed on a vertical plane to be perpendicular to each other.
On the other hand, a magnetic field producer 18 is disposed on the front surface 14 of the other shield machine 10a at a position corresponding to the reference point of the shield machine 10b. The magnetic field producer 18 is composed of a pair of magnetic field producing cables 18a and 18b forming rectangular loops which correspond to the horizontal directional eccentricity amount detection sensor 16a and the vertical directional eccentricity amount detection sensor 16b.
The magnetic field producing cables 18a and 18b are disposed in such a manner that their centers coincide with each other and they are perpendicular to each other. That is, the magnetic field producing cable 18a corresponding to the horizontal directional eccentricity amount detection sensor 16a is disposed in such a manner that its longitudinal direction is made to be in the vertical direction. The magnetic field producing cable 18b corresponding to the vertical directional eccentricity amount detection sensor 16b is disposed in such a manner that its longitudinal direction is made to be in the horizontal direction.
The position detection apparatus as described above is arranged in such a manner that electric currents (any of DCs, ACs or pulse currents may be used) are supplied to the magnetic field producing cables 18a and 18b so as to generate magnetic fields around them. The magnetic fields produced by the above-described cables 18a and 18b are formed into shapes directed in the forward direction of the shield machine 10a and symmetric between two major sides. Therefore, the magnetic field produced by the magnetic field producing cable 18a is detected by the horizontal directional eccentricity amount detection sensor 16a composed of a pair of magnetic sensors disposed on the horizontal plane in such a manner that they are perpendicular to each other. Then, the horizontal directional eccentricity amount of the boring 12 is detected in accordance with the difference in the intensity between the detected magnetic fields of the two magnetic sensors. Similarly, the vertical directional eccentricity amount of the boring 12 can be obtained by detecting the magnetic field produced by the magnetic field producing cable 18b by the vertical directional eccentricity amount detection sensor 16b.
However, since the position detection apparatus disclosed in Japanese Patent Application No. 1-223035 employs the rectangular loop cable as the magnetic field producer, the detection error becomes too large. That is, since the position detection apparatus disclosed in Japanese Patent Application No. 1-223035 utilizes technology of detecting a magnetic field produced by parallel cables having an infinite length or a length which is able to approximate it. Therefore, the magnetic field, which has been produced from another pair of the sides and which is not required to perform the detection, is also detected by the magnetic field detector, causing an error to be produced.
That is, in a case where the magnetic field producer can be assumed to be parallel cables having an infinite length, for example, in a case of a rectangular loop arranged in such a manner that the ratio b/a of the major side b and the minor side a is 100, the amount of eccentricity in a direction from the center of the major side and perpendicular to the major side is considered upon a result of a comparison made between the eccentricity amount x' obtained in accordance with the intensity of the magnetic field detected by the magnetic field detector and actual measurement result x. Since the inclination is 1 as shown in FIG. 30, the eccentricity amount x' obtained from the detected magnetic field becomes a value which is the same as the actual measurement result x.
However, if b/a is decreased, the intensity of the magnetic field detected by the magnetic field detector is affected by the magnetic field produced by the minor side a. As a result, the inclination is, as shown in FIG. 31, made to be smaller than 1 when the axis of abscissa stands for the actual measurement result x and the axis of ordinate stands for the eccentricity amount x' obtained in accordance with the intensity of the detected magnetic field. As a result, the eccentricity amount x' obtained from the magnetic field is made to be smaller than the actual measurement results x, causing the detection error to be generated.
Furthermore, since the magnetic field detector 16 is moved forward toward the magnetic field producer 18, the position detection sensor 16a, 16b are placed at a position away from the shield machine 10b. Therefore, if the boring 12 performs excavation in an inclined direction due to the change in the layer or the like, the above-described inclination cannot be detected. As a result, the positional deviation obtained by the magnetic field detector 16 does not coincide with the relative positional deviation between the two shield machines 10a and 10b. Therefore, a problem arises in that the positional deviation cannot accurately be detected. Similarly, a problem arises in a case where the two shield machines 10a and 10b make an inclined angle from each other.
Furthermore, the magnetic field producer 18 disclosed in Japanese Patent Application No. 1-223035 is arranged in such a manner that the magnetic field producing cables 18a and 18b are disposed on the front side cutter of the field machine 10a or the surface of the cutter bit attachment rotational base. Therefore, the magnetic field producing cables 18a and 18b come in contact with rocks and sediment and are thereby worn or disconnected when rocks or sediment are introduced into the shield machine 10a after the cutter or the cutter bit have rotated to excavate rocks or sediment. In order to prevent the above-described wear or the disconnection, the magnetic field producing cables 18a and 18b are disposed inside of the front surface of the shield machine 10a. However, an effective external magnetic field (that is, a leakage magnetic field) cannot easily be formed because the front surface of the shield machine 10a is made of steel, that is, magnetic material.